1. Field of the Invention
This invention relates to a device for the electrical determination of two-dimensional coordinates of a point, and more particularly to such a device in which a plate in the form of a resistance layer comprising a material having a given resistance characteristic carries contact strips, two of which strips may be, in each case, connected to different potentials of an electric voltage source and a potential which may be tapped at a point of the plate by means of a scanning device represents a gauge for the relative position of the point in relation to the contact strips.
2. Description of the Prior Art
A fundamental example of a typical device includes a plate which comprises a material having a high electrical resistance. Contact strips are arranged at the edges of the plate with a first pair of contact strips on the first parallel edges of the plate and a second pair of contact strips on other parallel edges of the plate perpendicular thereto. These contact strips can be connected to the poles of a voltage source so that, for example, a pair of contact strips have a voltage difference U in relation to one another. Consequently, there is a voltage difference U.sub.1 between a point on the plate and one of the two strips. This difference can be tapped with a scanning device, e.g. by means of a scanning probe. The voltage difference U.sub.1 represents a gauge for the relative position of the point in relation to the pair of contact strips. The device therefore operates in the manner of a potentiometer in which a resistance R has the voltage U applied thereto. The voltage U.sub.1 can be tapped from a portion R.sub.1 of the resistance R in accordance with the expression EQU U.sub.1 = U .sup.. R.sub.1 /R.
such a device involves the difficulty that the tapped potentials do not depend linearly upon the coordinates of a point on the plate. At a point which possesses the same distances from the contact strips of a first pair of strips, but which, however, lies closer to the contact strips of the second pair of strips, a different voltage difference is tapped. This effect occurs in that the contact strips of a second pair act as interferences, because of their high electrical conductivity, when the distance of the point from the first pair of contact strips is to be scanned. All the points of the plate in the vicinity of the second pair of contact strips carry approximately the same potential. If, however, contact strips having a high electrical resistance are used, the equivalent of a plate is formed which is contacted at points at four positions so that the potential tapped at the point in question also does not depend in a linear fashion upon the coordinates of that point.